The quality of an image that is acceptible for print is, of course, completely up to the eye of the beholder. However, the generally accepted Resolution for an image going to print is 300dpi.

The thing to remember is that even though you can "blow up" an image to a higher resolution/size, this does not mean that the computer can magically make the image bigger (like you see in movies: Could you enhance that?). The reality is that, despite being really good at guessing and filling in the gaps, computers can not make something out of nothing. If the pixels weren't there when you took the picture, the pixels won't be there later. So, if you do resize a low resolution image for print, it will likely look "soft" and have some jagged edges (known as Aliasing).